The present invention is generally related to signal processing, and, more particularly, to signal processing for controlling squelch in a radio receiver operable to scan at least a priority channel.
In Specialized Mobile Radios (SMRs), also generally known as LMR (Land Mobile Radio, PAMR (Public Access Mobile Radio), PMR (Private Mobile Radio), TMR (Trunked Mobile Radio), and TRS (Trunked Radio System), a squelch circuit is commonly used for automatically muting the audio response of a radio receiver configured to receive frequency modulated (FM) voice signals. That is, preventing annoying noise from being heard when there is no received signal present or the received signal is too weak to be intelligible, and unmuting the audio response when a signal likely to be intelligible is received.
Typical squelch circuits employ a measurement of the energy of a portion of the spectrum of the recovered audio (“out-of-band audio” that starts sufficiently above the highest audio frequency components, generally at about 3 kHz) as a factor in determining whether to mute the recovered voice signal. This is based on the well-known phenomenon that the energy in the out-of-band region decreases in response to a stronger radio signal. Unfortunately, known squelch circuits, when used on the priority channel of a frequency-scanned radio, typically require excessive processing time to determine the presence or absence of a signal. The excessive processing latency creates noticeable gaps in the speaker audio if the priority channel scan happens to interrupt an ongoing signal reception. Also, excessive priority channel squelch processing time can be detrimental if the priority channel is scanned during an ongoing reception of the control channel of a trunked system; in such cases, the processing time should be sufficiently less than the control channel slot time. Many of the earlier solutions rely on single speed techniques that require processing times on the order of 50 milliseconds or more before deciding if a signal is present or absent. During this processing interval, the audio is muted. This processing time would be fast enough if the radio is in a “standby” mode and not scanning. In fact, many conventional voice radio receivers that are not designed for channel scanning employ continuous detection of the energy of the out-of-band audio, often combined with hysteresis techniques, to avoid muting a recovered audio signal during brief periods of weak signal reception caused by fading. The response time of such circuits is typically 50 milliseconds (msec) or more, and they have provided reliable and efficient operation for non-scanning applications. However, such a processing time would be too slow for utilization in the priority channel for frequency-scanned operation for the reasons stated above.
U.S. Pat. No. 6,259,904 describes a technique that purportedly generates a fast squelch through various actions that require generating a noise signal from a demodulated signal, generating a squelch check request of one of two types, performing a release of a reset control in response to the squelch check request, generating an integrated noise signal having essentially no decay rate from the noise signal starting at the release of the reset control, generating a result of a comparison of the integrated noise signal to one of two values corresponding to the one of two types of squelch check requests at an expiration of a predetermined delay started at the release of the reset control, and controlling a muting of a speaker in response to the comparison. Thus, it appears that such technique requires relatively complex logic for implementing the above-described actions. In addition, such technique appears to use various circuit components, such as a resettable noise integrator consisting of a reset switch and logic for resetting a capacitor used for grounding the noise signal. Needless to say, such components add incremental costs and hardware complexity to the squelch circuit described in that patent.
Thus, it would be desirable to provide circuit and techniques, colloquially referred to as “Dual-Speed Squelch” or DSS techniques, that, at relatively low-cost, accurately and reliably provide sufficiently fast processing speed for priority scan operation in a radio terminal. It would be further desirable to minimize noticeable gaps in the speaker audio. A significant improvement over known techniques could be achieved if one could determine that there is no signal present and leave the priority channel in about 25% of the time of earlier squelch algorithms without having to use cumbersome logic or circuitry. It would be further desirable to provide circuit and techniques that enable high probability of detection for worst-case faded signal levels and low probability of falsing on nonexistent signals. It would be further desirable to provide an improved squelch technique that can be readily integrated or retrofitted into existing frequency-scanned radios so that such radios can provide improved performance without affecting the basic design of the radio.